As is known in the art, microwave systems, including microwave radar systems, can benefit from an amplifier whose gain can be adjusted in a predictable manner. For example, Group III-V (such as Gallium Nitride (GaN)) HFET amplifiers used in radars may have oscillation problems under certain conditions (e.g. temperature) due to excessive gain at a given condition (e.g. temperature).
As is known in the art, HFETs generally are formed by providing semiconductor layers of different materials forming a heterojunction. One such layer may be, for example, GaN and the other AlGaN to provide a high-electron mobility transistor (HEMT). The heterojunction supports a two-dimensional electron gas (2DEG) confined in a triangular quantum well (a potential well with only discrete energy values) at the heterojunction. This confinement of the 2DEG leads to quantized energy levels for motion along the channel of the HFET. Electrons confined to the heterojunction of HEMTs exhibit higher mobilities than those in MOSFETs, since the former utilizes an intentionally undoped channel thereby mitigating the deleterious effect of ionized impurity scattering.
As is also known in the art, the gain of a GaN HFET amplifier is set by the HFET transconductance (gm, the change in drain current divided by change in gate voltage) having a value fixed by the geometry and construction of the device and set by a fixed gate bias voltage applied at the gate above the channel of the transistor. One attempt to provide a variable gain of an FET amplifier uses two separate transistors in a cascode arrangement, such as described in a paper entitled “AlGaN/GaN-based Variable Gain Amplifiers for W-band Operation” by Diebold et al., Microwave Symposium Digest (IMS), 2013 IEEE MTT-S International DOI:10.1109/MWSYM.2013.6697340 publication year 2013 pages 1-4. However, the use of two separate transistors is relatively costly, lower yielding, and occupies a relatively large surface area.